The present invention relates in general to knocking control for an internal combustion engine. In particular, the invention concerns an ignition timing control apparatus for an internal combustion engine, which control apparatus includes a masking circuit for inhibiting a knocking detecting operation for a predetermined period beginning at a time point leading a basic ignition timing by a preselected crank angle.
In general, for the knocking control, there is established a so-called masking period during which a knocking detecting operation is inhibited, with a view to preventing a knocking detecting circuit from detecting noise ascribable to ignition erroneously as the knocking signal. The masking period begins at a basic or an actual ignition time point and has a predetermined duration. Although this system is immune to the false detection that the ignition noise produced upon or in succession to ignition is erroneously taken for the knocking signal, there still remains a possibility that those noises which are produced in precedence to the basic ignition timing due to the closing of inlet and/or exhaust valve of a cylinder other than the one in which the ignition is just to take place (this noise is referred to as the valve down noise) may be erroneously detected as the signal ascribable to the knocking. In this connection, with the terminology "basic ignition timing," it is intended to mean the ignition timing which is determined in dependence on the number of rotation and the negative intake pressure of the engine and which corresponds to a time point at which the output signal from the pick-up passes the zero point in transition from the positive to the negative direction in the case of an exemplary embodiment of the present invention, as will hereinafter be described by referring to FIG. 4 at A. It is known that the phase of the output signal of the pick-up is varied in dependence on the rotation number and the intake negative or vacuum pressure of the engine. Further, the terminology "actual ignition timing" is used to mean the time point at which the ignition spark is actually produced with a delay relative to the basic ignition timing due to the fact that the basic ignition timing is modified or corrected to lag by a lag crank angle in dependence on the knocking phenomenon as it occurs.